Calibration of infra red cameras

ABSTRACT

Described herein is an improved reference surface ( 10 ) which can be used for calibrating cooled infra red detectors. The reference surface ( 10 ) comprises a plurality of corner cube units ( 12, 14, 16, 18, 20, 22 ) which reflect radiation directly incident on them. The surface of each corner cube unit ( 12, 14, 16, 18, 20, 22 ) is partially reflective and partially emissive so that, when imaged by a cryogenically cooled detector, the reference surface ( 10 ) appears as a black body source at a lower temperature than the actual temperature thereof.

The present invention relates to improvements in or relating to thecalibration of infra red (IR) cameras and is more particularly concernedwith the calibration of such cameras employing cryogenically cooleddetector elements.

Such cameras typically employ a plurality of detector elements in theform of a focal plane array (FPA) on which an image of the scene isfocussed. However alternative configurations, such as a multi-elementlinear array, or a single element detector, over which an image of thescene is scanned, are also known.

Data from the detector element(s) may be used to create a visible imageon a display such as a TV monitor. This is analogous to a conventionalvisible light TV image, but where image intensity in different parts ofthe scene is a function of detected IR radiance, rather than visiblelight radiance from the corresponding part of the scene.

Individual detector elements exhibit variations in their sensitivity toincident radiation (flux), and in the offset, or “bias”, of theiroutputs as a function of time and operating conditions. The variationsin sensitivity may represent changes in the non-linearity in output as afunction of flux, and, as such, are dependent on flux level which,in-turn, depends on the temperatures of the elements comprising thescene being viewed. In cameras with a multi-element detector, thisvariability causes a non-uniform response to any given IR intensitylevel. This non-uniformity creates noise or artefacts across theresulting image which are highly undesirable.

Detectors may be required to produce numerical outputs which accuratelyrepresent the IR flux falling on the detector. This is also compromisedby the variations in detector performance.

It is known to calibrate IR cameras by taking readings from each elementwhen one or more reference surfaces at different temperatures arepresented to the camera. These readings are then used to calibrate thecamera. For robust and accurate calibration, it is necessary to presentsurfaces generating flux levels covering the range of those which may beencountered in the scenes to be imaged. However, the provision of asufficiently cold reference surface to generate the low flux levelsequivalent to scenes often encountered in practice requires substantivecooling of the reference surface. This can be expensive and difficult toachieve.

It is therefore an object of the present invention to provide acalibration system which overcomes the need to cool the referencesurface.

In accordance with one aspect of the present invention, there isprovided an infra red detector calibration system comprising a referencesurface which comprises a plurality of hollow corner cubes which arepartially reflective and partially emissive, temperature controllingmeans adapted to maintain the reference surface at a desired calibrationtemperature, processing means for receiving an output signal generatedby an infrared detector at said the desired calibration temperature,comparing said detector output signal with a predetermined ideal outputsignal for said desired calibration temperature and calculating acalibration coefficient on the basis of the difference between thedetector output signal and the ideal output signal at said desiredcalibration temperature.

In accordance with another aspect of the present invention, there isprovided a method of calibrating an infra red detector comprising aplurality of detector elements, the method comprising:

-   -   presenting a reference surface at a known temperature to an        infra red detector;    -   measuring the output of each detector element;    -   comparing the measured output of each detector element with a        nominal output for the known reference surface temperature to        determine a calibration error at the known temperature; and    -   heating the reference surface to one or more further known        temperatures and repeating steps b) and c) to determine a        calibration errors for each of the further known temperatures.

For a better understanding of the present invention, reference will nowbe made, by way of example only, to the accompanying drawings in which:

FIG. 1 illustrates a portion of a reflective surface in accordance withthe present invention;

FIG. 2 illustrates a portion of one of the sectors of the reflectivesurface in more detail;

FIG. 3 illustrates the operation of a corner cube as it reflects light;

FIG. 4 illustrates an incoming beam of light being reflected by athree-dimensional corner cube; and

FIG. 5 illustrates a block diagram of calibration apparatus utilising areference surface in accordance with the present invention.

In accordance with the present invention, a reference surface isprovided which, when viewed by a cryogenically cooled detector, canproduce the radiance equivalent to that of a lower temperature blackbody. This allows calibration of an infra red (IR) camera for coldscenes without the necessity of cooling the surface to the actual blackbody temperature.

The present invention provides a partially reflective, partiallyemissive surface which presents the equivalent radiance of a relativelycold back body, for example, at 0° C., at equipment operatingtemperatures, typically at room temperature or above. The surface can beused for calibration of a cooled IR focal plane array (FPA) detector.

A simple plain surface of N % emissivity reflects surrounding hotmetalwork at 100-N %, such that it is impossible to achieve a lowoverall radiance simply by altering the emissivity of the surface. To doso, it is necessary to cool the surface. By utilising a corner-cubepatterned surface, it is possible to substantially eliminate reflectionsoff hot (or ambient temperature) metalwork, reflecting only thetemperature of the cold FPA which is typically at 77K. As N % of thepatterned surface is still emissive, it is possible to control theequivalent temperature of the surface (above 77K) by heating thesurface.

Turning now to FIG. 1, a portion 10 of a reference surface is shown. Thereference surface is formed by a pattern of hollow corner cubes 12, 14,16, 18, 20, 22 and is similar to that found in bicycle reflectors. Eachsurface of each of the cubes 12, 14, 16, 18, 20, 22 is silvered oraluminised to present a nominally 100% reflective surface. This ensuresthat a FPA (not shown) to be calibrated receives reflected light onlyfrom the cold FPA itself, the flux therefrom being small when comparedwith the desired flux. The temperature of the FPA is controlled ataround 77K. The reflective surface is then overlaid with a uniformpattern of matt black paint with an average density of N % such that theeffective emissivity is N %. The paint pattern and ideally the surfacestructure must be at a relatively high spatial frequency and positionedsuch that de-focus through the optics of the camera presents a uniformintensity at the FPA. Such an arrangement is shown in FIG. 2.

FIG. 2 illustrates a portion 24 of the reference surface in more detail.Here, it can be seen that matt black paint 26 surrounds a plurality ofdots 28 which are effectively uncoated by the paint 26. These dots 28are what is left of the original reflective surface after the matt blackpaint 26 has been applied.

After silvering, or aluminising, and subsequent painting, when viewed bya cooled IR detector the whole reference surface appears as a black bodysource at a temperature which is lower than the actual temperature ofthe reference surface. The apparent temperature is a function of theactual temperature of the reference surface, since the radiance from thematt black areas increases as they are heated. This means that thereference surface can be heated and temperature controlled to providedifferent equivalent reference temperatures (above 77K) for the FPA toview during calibration.

FIG. 3 illustrates a cross-section of a corner cube 30. As shown, anincoming beam on path 32 is reflected to follow path 34 and is thenreflected again to follow path 36. Similarly, an incoming beam on path38 is reflected onto path 40 and then reflected onto path 42. In bothinstances, a beam on the incoming paths 32, 38 is reflected by thecorner cube 30 to exit on respective parallel paths 36, 42, in theopposite direction to the incoming beams. This is termedretro-reflection.

It will readily be understood that the principles described above withreference to FIG. 3, apply to all light beams entering athree-dimensional corner cube, for example, cubes 12, 14, 16, 18, 20, 22as described with reference to FIG. 1 above. In particular, FIG. 4illustrates corner cube surfaces which are aligned parallel to some axesx, y, z. An incoming beam of light 44 has motion in some direction (i.e.has a velocity) with components, say x₁, y₁, z₁. The beam 44 is alwaysreflected once by each of the three surfaces (x=y=0; x=z=0; and y=z=0).At each surface, the sign of the component along the normal to thesurface is inverted, e.g. at the surface x=y=0, the component ofvelocity z₁ is inverted and becomes −z₁. The other components are notchanged. After all three such reflections, the reflected beam 44′ has avelocity (direction) −x₁, −y₁, −z₁. This is equal and opposite to thatof the incoming beam 44.

In the manufacture of the reference surface described above, it isassumed that silvering or aluminising prior to painting produces areflective surface which is more accurate geometrically, than performingthis process in reverse.

As an alternative to painting, the silvered or aluminised surface can beetched to leave a non-reflective surface underneath. This will have anequivalent effect to applying a matt black paint surface.

FIG. 5 illustrates a calibration arrangement 50 which utilises areference surface 52 as described above with reference to FIGS. 1 to 4.The reference surface 52 is connected to a heater unit 54 which heats itto the calibration temperatures at which an infra red camera 56 is to becalibrated. The heater unit 54 is connected to a controller 58 whichcontrols the arrangement 50. A temperature sensor on the referencesurface 52 feeds back temperature data to the heater controller 58 tomaintain the temperature of the reference surface 52 thermostatically atthe desired temperature. The desired temperature is defined by aprocessor 62, as part of the calibration sequence, and transmitted tothe controller 58.

As is well known, the camera 56 is connected to a cooling control unit60 which operates to cool the detector elements (not shown) in thecamera 56, and to the processor 62 which receives signals from thecamera 56, processes the signals and then provides an output 64. Theprocessor 62 is also operable for controlling the cooling control unit60 so that the camera 56 is maintained within its optimum operatingtemperature range during operation and calibration.

The processor 62 is also connected to a memory unit 66 where thecalibration coefficients determined during calibration are stored.

During calibration, the reference surface 52 is positioned so as to beviewed by the camera 56. Initially, the heater unit 54 is notoperational and the reference surface 52 produces a flux below theminimum required calibration flux level. This flux is principally acombination of reflected flux from the cooled detector, and the emittedflux from the emissive parts of the reference surface, which are at theinternal ambient temperature of the camera. There will also be somestray flux due to imperfections in the reflectors and reflections fromthe camera optics, but this can be minimised by a carefully chosendesign.

The heater unit 54 is then activated under the control of the controller58 to raise the temperature of the reference surface 52 to that requiredto generate the minimum required calibration flux level. The camera 56detects the flux presented by the reference surface 52, and providessignals representing the outputs of each detector element to theprocessor 62. As is usual, the processor 62 processes the signal andprovides output 64, representing the image intensity at each part of thescene. Within processor 62 the output value from each detector element(included in processor output 64) is compared with the ideal valuecalculated for the current reference surface temperature. Anydifferences between the output value and the ideal value for eachdetector element are passed to the memory unit 66 for storing untilneeded later in the calibration process.

The heater unit 54 is then activated under the control of the controller58 to further raise the temperature of the reference surface 52. Thisnew elevated flux level generated by the reference surface 52 isprimarily a combination of reflected flux from the camera 56 and theemissive parts of the reference surface 52 at the elevated temperaturecontrolled by the heater unit 54. Again, within processor 62, the outputvalue from each detector element is compared with the ideal output valuefor the current reference surface temperature. Any differences are againpassed to the memory unit 66 for storing until needed later in thecalibration process. This process is repeated for each of severalcalibration temperatures until the memory unit 66 contains sufficientdata to calculate all the calibration coefficients necessary for thecamera 56 to operate effectively. Typically such readings are taken ateach of three different reference surface temperatures. The storedoutput errors are then used within processor 62 to calculate thecoefficients of a second-order polynomial function relating outputerrors to scene temperature for each detector element.

It will readily be understood that, due to the construction of thereferences surface 52 as described above with reference to FIGS. 1 to 4ideally the only flux reflected into the camera is that originating fromthe cooled components of the camera detector itself. This preventstemperatures of the environment surrounding the camera 56 fromsignificantly interfering with the calibration process. Moreover, thetemperatures of the reference surface 52 can be accurately controlled bycontrolling the amount of heat applied thereto by the heating unit 54.

Naturally the camera 56, cooling control unit 60, processor 62 andmemory unit 62 may be housed as a single unit as shown by the dottedline 66.

It should also be noted that the method described above can be used witha reference surface which can be both heated and cooled. In this casethe required degree of cooling required for the reference surface may besubstantially reduced.

1. An infra red detector calibration system comprising a referencesurface which comprises a plurality of hollow corner cubes which arepartially reflective and partially emissive, temperature controllingmeans adapted to maintain the reference surface at a desired calibrationtemperature, processing means for receiving an output signal generatedby an infrared detector at said the desired calibration temperature,comparing said detector output signal with a predetermined ideal outputsignal for said desired calibration temperature and calculating acalibration coefficient on the basis of the difference between thedetector output signal and the ideal output signal at said desiredcalibration temperature.
 2. An infra red detector calibration systemaccording to claim 1 wherein the emissivity of the reference surface iscontrolled by controlling the temperature of said reference surface. 3.An infra red detector calibration system, according to claim 1, whereineach corner cube comprises a reflective surface and a matt surface toform an effective surface emissivity of N %.
 4. An infra red detectorcalibration system according to claim 3, wherein the reflective surfacecomprises a silvered surface.
 5. An infra red detector calibrationsystem according to claim 3, wherein the reflective surface comprises analuminized surface.
 6. An infra red detector calibration systemaccording to claim 3, wherein the matt surface comprises a matt blackpaint overlying the reflective surface.
 7. An infra red detectorcalibration system according to claim 3, wherein the matt surfacecomprises a non-reflective surface etched into the reflective surface.8. A method of calibrating an infra red detector comprising a pluralityof detector elements using a reference surface, the method comprising:a) presenting the reference surface at a known temperature to an infrared detector; b) measuring the output of each detector element; c)comparing the measured output of each detector element with a nominaloutput for the known reference surface temperature to determine acalibration error at the known temperature; and d) heating the referencesurface to one or more further known temperatures and repeating steps b)and c) to determine a calibration errors for each of the further knowntemperatures.
 9. A method according to claim 8, further comprising thestep of calculating a function relating the output error of eachdetector element to the temperature of the reference surface.
 10. Amethod according to claim 9, wherein the function is a polynominalfunction.
 11. A method according to claim 8, further comprising the stepof storing the calibration constants for application to readingsobtained from the detector. 12-13. (canceled)